Although plasma display panels are known for many years, plasma displays are encountering a growing interest from TV manufacturers. Indeed, this technology now makes it possible to achieve flat colour panels of large size and with limited depths without any viewing angle constraints. The size of the displays may be much larger than the classical CRT picture tubes would have ever been allowed.
Referring to the latest generation of European TV sets, a lot of work has been made to improve its picture quality. Consequently, there is a strong demand, that a TV set built in a new technology like the plasma display technology has to provide a picture so good or better than the old standard TV technology. On one hand, the plasma display technology gives the possibility of nearly unlimited screen size, also of attractive thickness, but on the other hand, it generates new kinds of artefacts which could reduce the picture quality. Most of these artefacts are different from the known artefacts occurring on classical CRT color picture tubes. Already due to this different appearance of the artefacts makes them more visible to the viewer since the viewer is used to see the well-known old TV artefacts.
In the plasma display technology field a specific new artefact is known, which is called “dynamic false contour effect” since it corresponds to disturbances of gray levels and colors in the form of an apparition of colored edges in the picture when an observation point on the matrix screen moves. This kind of artefact is enhanced when the image has a smooth gradation like when the skin of a person is being displayed (e. g. displaying of a face or an arm, etc.). In addition, the same problem occurs on static images when observers are shaking their heads and that leads to the conclusion that such a failure depends on the human visual perception and happens on the retina of the eye.
Two approaches have been discussed to compensate for the false contour effect. As the false contour effect is directly related to the sub-field organization of the used plasma technology one approach is to make an optimization of the sub-field organization of the plasma display panels. The sub-field organization will be explained in greater detail below but for the moment it should be noted that it is a kind of decomposition of the 8-bit gray level in 8 or more lighting sub-periods. An optimization of such a picture encoding will have, indeed, a positive effect on the false contour effect. Nevertheless, such a solution can only slightly reduce the false contour effect amplitude but in any cases the effect will still occur and will be perceivable. Furthermore, sub-field organization is not a simple matter of design choice. The more sub-fields are allowed the more complicated will the plasma display panel be. So, optimization of the sub-field organization is only possible in a narrow range and will not eliminate this effect alone.
The second approach for the solution of above-mentioned problem is known under the expression “pulse equalization technique”. This technique is a more complex one. It utilizes equalizing pulses which are added or separated from the TV signal when disturbances of gray scales are foreseen. In addition, since the fact that the false contour effect is motion relevant, we need different pulses for each possible speed. That leads to the need of a big memory storing a number of big look-up tables (LUT) for each speed and there is a need of a motion estimator. Furthermore, since the false contour effect depends on the sub-field organization, the pulses have to be re-calculated for each new sub-field organization. However, the bid disadvantage of this technique results from the fact that the equalizing pulses add failures to the picture to compensate for a failure appearing on the eye retina. Additionally, when the motion is increasing in the picture, there is a need to add more pulses to the picture and that leads to conflicts with the picture contents in case of very fast motion.
The invention deals with a specific new problem which is called “appearing area” since it corresponds to missing information for controlling pixels of a display by shifting time periods from one pixel to another pixel for compensating dynamic false contour effects.
In a first approach there has been disclosed a method for compensating the false contour effect using a motion estimator which determines motion vectors for the pixels. The resulting motion vectors are utilized for re-coding the pixels of the block wherein in the re-coding step a step of shifting the time periods of pixels is included. The time periods define the time during which the pixels are activated for sending out light. The time periods are hereinafter also called “sub-fields”. The so calculated data for activating the pixels are used to display the picture instead of displaying the original pixel data.
There are situations in which a block and a background are moving in different directions and therefore the shifting of the sub-field code word entries for the pixels of the moving front object and the moving background object in the two different directions generates a lack of light pulses for pixels of the appearing area.
As a conclusion the shifting of sub-fields as it is disclosed in the document EP 0 978 817 A1 generates in certain situations mistakes in the video pictures.